Ultrasonic flowmeter and flow rate calculation method

Abstract

To enable measurement errors to be reduced compared to before, even when flow rate measurement is conducted using a zero-cross point.SOLUTION: The present invention comprises: a received signal acquisition unit 401 and a received signal acquisition unit 402 for acquiring a received signal; a zero-cross point measurement unit 403 and a zero-cross point measurement unit 404 for measuring the time till a zero-cross point multiple times for each of a plurality of unit measurement steps; a time difference calculation unit 405 for calculating a time difference between measurement results from the zero-cross point measurement unit 403 and measurement results from the zero-cross point measurement unit 404; an average value calculation unit 406 for calculating the average value of time differences at the first-half zero-cross point; an average value calculation unit 407 for calculating the average value of time differences at the latter-half zero-cross point; a difference calculation unit 408 for calculating the difference between calculation results from the average value calculation unit 406 and calculation results from the average value calculation unit 407 as a correction value; and a flow rate computation unit 409 for computing the flow rate of a fluid which is the object to be measured, on the basis of calculation results from the time difference calculation unit 405 and the correction value calculated by the difference calculation unit 408.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an ultrasonic flowmeter that measures a flow rate by using ultrasonic waves and a flow rate calculation method. [Background technology]

[0002] 2. Description of the Related Art Conventionally, ultrasonic flowmeters are known that measure the flow rate of a fluid to be measured based on the difference in propagation time between ultrasonic waves transmitted and received by a pair of ultrasonic sensors. Regarding such ultrasonic flowmeters, a method is known in which a predetermined number of zero-crossing points of a received signal are detected, the propagation time of the ultrasonic waves to the zero-crossing points is measured, and the flow rate of the fluid is calculated based on the propagation time (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2012-242091 A Summary of the Invention [Problem to be solved by the invention]

[0004] Here, due to differences in sensor characteristics between a pair of ultrasonic sensors, there is a phenomenon in which the zero point of the time difference (propagation time difference) used in flow measurement drifts with temperature. The amount of variation in this drift differs depending on the zero cross point. Therefore, when flow measurement is performed using zero cross points at different locations in the received signal as in the conventional technology, a zero cross point with a large amount of drift variation is used, which may result in a large measurement error.

[0005] The present invention has been made to solve the above-mentioned problems, and aims to provide an ultrasonic flowmeter that can reduce measurement errors compared to conventional methods, even when flow rate measurement is performed using zero crossing points. [Means for solving the problem]

[0006] The ultrasonic flowmeter according to the present invention includes a first reception signal acquisition unit that acquires a reception signal received by one ultrasonic sensor, a second reception signal acquisition unit that acquires a reception signal received by the other ultrasonic sensor, a first zero-cross point measurement unit that measures the time from the start of transmission to the zero-cross point multiple times for each of multiple unit measurement steps based on the acquisition results by the first reception signal acquisition unit, a second zero-cross point measurement unit that measures the time from the start of transmission to the zero-cross point multiple times for each of multiple unit measurement steps based on the acquisition results by the second reception signal acquisition unit, and a zero-cross point measurement unit that calculates the time from the start of transmission to the zero-cross point multiple times for each of multiple unit measurement steps based on the measurement results by the first zero-cross point measurement unit. The measurement device is characterized by comprising a time difference calculation unit that calculates a time difference with the measurement result by the zero cross point measurement unit, a first average value calculation unit that calculates an average value of the time differences at the first zero cross points based on the calculation result by the time difference calculation unit, a second average value calculation unit that calculates an average value of the time differences at the second zero cross points based on the calculation result by the time difference calculation unit, a difference calculation unit that calculates the difference between the calculation result by the first average value calculation unit and the calculation result by the second average value calculation unit as a correction value, and a flow rate calculation unit that calculates the flow rate of the fluid to be measured based on the calculation result by the time difference calculation unit and the correction value calculated by the difference calculation unit. Effect of the Invention

[0007] According to the present invention, since it is configured as described above, even when flow rate measurement is performed using the zero crossing point, it is possible to reduce measurement errors compared to the conventional art. [Brief description of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a configuration example of an ultrasonic flowmeter according to a first embodiment. [Diagram 2] FIG. 2 is a diagram illustrating an example of the configuration of a calculation unit according to the first embodiment. [Diagram 3] FIG. 2 is a diagram for explaining zero crossing points. [Figure 4] 5 is a flowchart showing an example of the operation of a calculation unit in the first embodiment. [Diagram 5]FIG. 13 is a diagram for explaining the drift of the zero point due to the time difference between each zero crossing point. [Figure 6] FIG. 4 is a diagram showing an example of various parameters used in a calculation unit in the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Embodiment 1 FIG. 1 is a diagram showing a configuration example of an ultrasonic flowmeter according to a first embodiment. An ultrasonic flowmeter measures a fluid by using ultrasonic waves. This ultrasonic flowmeter includes a measuring tube 1, ultrasonic sensors 2 and 3, and a calculation unit 4, as shown in FIG.

[0010] The measuring tube 1 is a cylindrical member through which a fluid to be measured flows.

[0011] The ultrasonic sensor 2 is an ultrasonic transducer that is attached to the upstream side of the side wall of the measurement pipe 1 and transmits and receives ultrasonic waves between it and the ultrasonic sensor 3 inside the measurement pipe 1. That is, the ultrasonic sensor 2 transmits ultrasonic waves to the downstream side (ultrasonic sensor 3) inside the measurement pipe 1, and receives ultrasonic waves from the downstream side (ultrasonic sensor 3) as a reception signal.

[0012] The ultrasonic sensor 3 is an ultrasonic transducer that is attached to the downstream side of the side wall of the measurement pipe 1 and transmits and receives ultrasonic waves between it and the ultrasonic sensor 2 within the measurement pipe 1. That is, the ultrasonic sensor 3 transmits ultrasonic waves to the upstream side (ultrasonic sensor 2) within the measurement pipe 1, and receives ultrasonic waves from the upstream side (ultrasonic sensor 2) as a reception signal.

[0013] The positional relationship between the ultrasonic sensors 2 and 3 is designed according to the propagation paths of the ultrasonic waves used by the ultrasonic sensors 2 and 3.

[0014] The calculation unit 4 calculates the flow rate of the fluid in the measuring pipe 1 based on the results of transmission and reception by the ultrasonic sensor 2 and the results of transmission and reception by the ultrasonic sensor 3.

[0015] As shown in FIG. 2, this calculation unit 4 includes a received signal acquisition unit (first received signal acquisition unit) 401, a received signal acquisition unit (second received signal acquisition unit) 402, a zero cross point measurement unit (first zero cross point measurement unit) 403, a zero cross point measurement unit (second zero cross point measurement unit) 404, a time difference calculation unit 405, an average value calculation unit (first average value calculation unit) 406, an average value calculation unit (second average value calculation unit) 407, a difference calculation unit 408 and a flow rate calculation unit 409.

[0016] The calculation unit 4 is realized by a processing circuit such as an integrated circuit (IC) or a system large scale integration (LSI), or a central processing unit (CPU) that executes a program stored in a memory or the like.

[0017] The received signal acquisition unit 401 acquires the received signal received by the ultrasonic sensor 2 .

[0018] The received signal acquisition unit 402 acquires the received signal received by the ultrasonic sensor 3 .

[0019] The zero-crossing point measuring unit 403 measures the time from the start of transmission to the zero-crossing point multiple times based on the acquisition result by the received signal acquiring unit 401. The zero-crossing point measuring unit 403 performs the above process for each of the multiple received signals (each unit measurement step). As shown in FIG. 3, for example, a zero-cross point is a point where the strength of a received signal becomes zero after the strength of the received signal exceeds a threshold (threshold voltage) after reception starts. Usually, the zero-cross point is measured at a point between the reception start point of the received signal and the point where the received signal has a maximum amplitude. In FIG. 3, the upper part shows the transmission waveform of the ultrasonic wave, and the lower part shows the reception waveform of the ultrasonic wave (waveform of the received signal). In FIG. 3, reference numeral 31 indicates the reception start point, reference numeral 32 indicates the threshold, reference numeral 33 indicates the zero-cross point, and reference numeral 34 indicates the point where the received signal has a maximum amplitude. The number of zero-cross points at which the zero-cross point measuring unit 403 measures time in one unit measurement step is set in advance. The number of unit measurement steps is set in advance.

[0020] The zero-crossing point measuring unit 404 measures the time from the start of transmission to the zero-crossing point multiple times based on the acquisition result by the received signal acquiring unit 402. The zero-crossing point measuring unit 404 performs the above process for each of the multiple received signals (each unit measurement step). The number of zero crossing points at which the zero crossing point measuring section 404 measures time in one unit measurement step is set in advance, and the number of unit measurement steps is also set in advance.

[0021] The operations of the received signal acquisition units 401 and 402 and the zero-crossing point measurement units 403 and 404 can be realized by one circuit system. In other words, the above operations can be realized by switching the connection between the above circuits and the ultrasonic sensors 2 and 3 depending on whether transmission and reception is in the forward direction or the reverse direction.

[0022] Time difference calculation unit 405 calculates the time difference between the measurement result by zero cross point measurement unit 403 and the measurement result by zero cross point measurement unit 404. At this time, first, time difference calculation unit 405 calculates an average value by averaging the measurement results in each unit measurement step by zero cross point measurement unit 403 for each zero cross point. Similarly, time difference calculation unit 405 calculates an average value by averaging the measurement results in each unit measurement step by zero cross point measurement unit 404 for each zero cross point. Then, time difference calculation unit 405 calculates the difference between the above two average values ​​for each zero cross point, thereby calculating the time difference at each zero cross point.

[0023] Based on the calculation result by time difference calculation section 405, average value calculation section 406 calculates the average value of the time differences at the first half zero crossing points. In addition, the first half zero crossing point refers to one or more zero crossing points that are close to the reception start point when multiple zero crossing points that occur from the reception start point of the received signal to the point where the received signal has the maximum amplitude are arranged in chronological order.

[0024] Based on the calculation result by time difference calculation section 405, average value calculation section 407 calculates the average value of the time differences at the latter half zero crossing points. In addition, the latter zero crossing point refers to one or more zero crossing points that are farther from the reception start point when multiple zero crossing points that occur from the reception start point of the received signal to the point where the received signal has the maximum amplitude are arranged in chronological order, and is a zero crossing point that occurs after the first zero crossing point.

[0025] The difference calculation section 408 calculates the difference between the average value calculated by the average value calculation section 406 and the average value calculated by the average value calculation section 407 as a correction value.

[0026] Flow rate calculation unit 409 calculates the flow rate of the fluid in measuring tube 1 based on the calculation result by time difference calculation unit 405 and the calculation result by difference calculation unit 408. At this time, flow rate calculation unit 409 first corrects the time difference calculated by time difference calculation unit 405 with the correction value calculated by difference calculation unit 408. Then, flow rate calculation unit 409 calculates the flow rate of the fluid based on the time difference corrected with the correction value (corrected time difference). The operating principle of flow rate calculation unit 409 can adopt the conventional flow rate calculation principle except for the use of the corrected time difference, and a description thereof will be omitted.

[0027] Next, an example of the operation of the calculation unit 4 in the first embodiment shown in FIG. 2 will be described with reference to FIG. Here, the pair of ultrasonic sensors 2, 3 have a phenomenon in which the zero point of the time difference (propagation time difference) used in the fluid measurement drifts with temperature due to differences in sensor characteristics. If the fluctuation width of this drift is within a predetermined range, the measurement accuracy of the ultrasonic flowmeter is satisfied. On the other hand, as shown in Figure 5, when the time difference used in the above flow measurement is broken down into zero crossing points, it can be seen that the later the zero crossing point on the time axis is, the greater the influence of temperature change becomes.

[0028] In flow rate measurement with an ultrasonic flowmeter, if only the time difference of the first zero crossing point is used, systematic errors (such as zero point drift due to temperature changes) will be small, but the signal-to-noise ratio will be poor, and the number of zero crossing points that can be used to calculate the average will be reduced, resulting in large random errors. SN is the ratio of signal (S) to noise (N). Random errors are caused by chance and cannot be controlled by the person making the measurement, and their effects can be reduced by, for example, increasing the number of measurements. On the other hand, in flow rate measurement using an ultrasonic flowmeter, if the time difference of the latter zero crossing point is used, the signal-to-noise ratio is good and the random error is small, but the systematic error becomes large.

[0029] Therefore, in the ultrasonic flowmeter according to the first embodiment, the systematic error is quantified from the time difference at the first zero crossing point, and the time difference at the second zero crossing point is corrected using the quantified value, and then the flow rate is measured. As a result, in the ultrasonic flowmeter according to the first embodiment, the systematic error can be reduced while keeping the random error low.

[0030] In an example of the operation of the calculation unit 4 in the first embodiment shown in FIG. 2, first, the received signal acquisition unit 401 and the received signal acquisition unit 402 acquire a received signal as shown in FIG. 4 (step ST401). That is, the received signal acquisition unit 401 acquires the received signal received by the ultrasonic sensor 2 . Similarly, the received signal acquisition unit 402 acquires the received signal received by the ultrasonic sensor 3 .

[0031] Next, zero-crossing point measuring section 403 and zero-crossing point measuring section 404 measure the time from the start of transmission to the zero-crossing point multiple times for each unit measurement step (step ST402). That is, the zero-crossing point measuring unit 403 measures the time from the start of transmission to the zero-crossing point multiple times for each unit measurement step, based on the results acquired by the received signal acquiring unit 401. Similarly, the zero-crossing point measuring unit 404 measures the time from the start of transmission to the zero-crossing point multiple times for each unit measurement step, based on the results acquired by the received signal acquiring unit 402 .

[0032] 3 shows a case where the zero-cross point measuring section 403 and the zero-cross point measuring section 404 measure the times of six zero-cross points in one unit measurement process. The number of unit measurement processes is, for example, 31.

[0033] The time of the m-th zero cross point in the k-th unit measurement step measured by the zero cross point measuring section 403 is represented as ZCm(k). Moreover, the time of the m-th zero cross point in the k-th unit measurement step measured by the zero cross point measuring section 404 is represented as inverse ZCm(k).

[0034] Next, time difference calculation unit 405 calculates the time difference (ZCmΔt) between the measurement result by zero-cross point measurement unit 403 and the measurement result by zero-cross point measurement unit 404 as shown in the following equation (1) (step ST403). At this time, first, time difference calculation unit 405 calculates an average value by averaging the measurement results in each unit measurement step by zero-cross point measurement unit 403 for each zero-cross point. Similarly, time difference calculation unit 405 calculates an average value by averaging the measurement results in each unit measurement step by zero-cross point measurement unit 404 for each zero-cross point. Then, time difference calculation unit 405 calculates the difference between the above two average values ​​for each zero-cross point, thereby calculating the time difference at each zero-cross point. ZCmΔt=(ΣReverse ZCm(k) / k)-(ΣOrder ZCm(k) / k) (1)

[0035] Next, average value calculation section 406 calculates the average value of the time differences at the first half zero crossing points based on the calculation result by time difference calculation section 405 (step ST404). At this time, for example, average value calculation section 406 calculates the average value (Δt1_2_3_4) of the time differences at the first to fourth zero crossing points as shown in the following formula (2). Note that, in order to maintain the standard deviation, average value calculation section 406 may calculate the final average value (Δt1_2_3_4_N) by performing N averaging, that is, by repeating the calculation of the above average value N times (multiple times) and averaging. N is a number sufficient to reduce the influence of random errors. Δt1_2_3_4=(ZC1Δt+ZC2Δt+ZC3Δt+ZC4Δt) / 4 (2)

[0036] Next, average value calculation section 407 calculates the average value of the time differences at the latter zero crossing points based on the calculation result by time difference calculation section 405 (step ST405). At this time, for example, average value calculation section 407 calculates the average value (Δt5_6) of the time differences at the fifth and sixth zero crossing points as shown in the following formula (3). Note that, in order to maintain the standard deviation, average value calculation section 407 may calculate the final average (Δt5_6_N) by performing N averaging, that is, by repeating the calculation of the above average value N times (multiple times) and averaging. N is a number sufficient to reduce the influence of random errors. Δt5_6=(ZC5Δt+ZC6Δt) / 2 (3)

[0037] Next, difference calculation section 408 calculates the difference between the average value calculated by average value calculation section 406 and the average value calculated by average value calculation section 407 as a correction value (step ST406). At this time, in the above example, difference calculation section 408 calculates the difference value (correction value) by subtracting the average value of the time differences at the first to fourth zero crossing points from the average value of the time differences at the fifth and sixth zero crossing points, as shown in the following equation (4). This correction value is a correction value for reducing a systematic error in the time differences at the latter zero crossing points. Correction value = Δt5_6_N-Δt1_2_3_4_N (4)

[0038] Next, flow rate calculation unit 409 calculates the flow rate of the fluid in measuring tube 1 based on the calculation result by time difference calculation unit 405 and the calculation result by difference calculation unit 408 (step ST407). At this time, in the above example, flow rate calculation unit 409 first calculates the average value of the time differences at the fifth and sixth zero crossing points after correction (corrected Δt5_6) by subtracting the correction value from the average value of the time differences at the fifth and sixth zero crossing points as shown in the following formula (5). In this way, flow rate calculation unit 409 can correct the temperature characteristics of ZC5Δt and ZC6Δt to approach the temperature characteristics of ZC1Δt to ZC4Δt while maintaining the standard deviations of ZC5Δt and ZC6Δt. Then, flow rate calculation unit 409 calculates the average value of the time differences at the first and second zero crossing points, the average value of the time differences at the third and fourth zero crossing points, and the average value of the corrected average values ​​of the time differences at the fifth and sixth zero crossing points to calculate the corrected time difference (corrected Δt) as shown in the following formula (6). Then, flow rate calculation unit 409 uses this corrected time difference to calculate the flow rate of the fluid. After correction Δt5_6 = Δt5_6 - correction value (5) Corrected Δt = (Δt1_2 + Δt3_4 + corrected Δt5_6) / 3 (6)

[0039] FIG. 6 shows an example of the values ​​of various parameters used in the calculation unit 4 in the first embodiment.

[0040] In the above example, the calculation unit 4 obtains the correction value using Δt5_6 and Δt1_2_3_4. However, the calculation unit 4 may obtain the correction value using the time difference at the other first half zero crossing points and the time difference at the second half zero crossing points. In other words, the number of zero crossing points used by the calculation unit 4, the number of zero crossing points included in the first half zero crossing points, and the number of zero crossing points included in the second half zero crossing points are not limited to the above example. In addition, the calculation unit 4 can reduce the error of the zero crossing points caused by the shift of the reference potential (0 point) and the change of the signal strength (slope) by averaging the rise-fall pairs (an even number of zero crossing points).

[0041] In this way, the ultrasonic flowmeter according to the first embodiment compares the time difference at the first zero crossing point with the time difference at the second zero crossing point, and performs correction so that the systematic error at the second zero crossing point is at the same level as the systematic error at the first zero crossing point. This makes it possible to reduce the overall systematic error in the ultrasonic flowmeter according to the first embodiment.

[0042] It is also possible to increase the number of unit measurement steps and use the time difference at the first zero crossing point to reduce systematic errors (such as zero point drift due to temperature changes) while maintaining the measurement accuracy (random errors). However, increasing the number of unit measurement steps increases power and memory consumption. In contrast, in the ultrasonic flowmeter according to embodiment 1, in order to prevent an increase in the random error without increasing the number of unit measurement steps, it is necessary to keep the random error of the time difference at the latter zero crossing point after correction at the same level as the random error of the time difference at the latter zero crossing point before correction. On the other hand, the relationship between the time difference at the first zero crossing point and the time difference at the second zero crossing point does not change in a short period of time. In other words, in a short period of time (during N averaging), the difference between the time difference at the first zero crossing point and the time difference at the second zero crossing point does not change significantly. Therefore, N averaging can be used.

[0043] Therefore, in the ultrasonic flowmeter according to the first embodiment, the difference between the time difference at the first zero crossing point and the time difference at the second zero crossing point is averaged over a fixed period of time. This makes it possible to reduce accidental errors in the time difference at the second zero crossing point after correction.

[0044] For example, let A be the standard deviation of Δt1_2_3_4 and B be the standard deviation of Δt5_6. In this case, the standard deviation of Δt1_2_3_4 when the average number of times is N is A / (N)^0.5. Also, when the average number of times is N, the standard deviation of Δt5_6 is B / (N)^0.5.

[0045] In this case, the correction value is Δt5_6_N-Δt1_2_3_4_N, so the standard deviation of the correction value is ((B / (N)^0.5)^2+(A / (N)^0.5)^2)^0.5. In addition, since corrected Δt5_6 is Δt5_6-the correction value, the standard deviation of corrected Δt5_6 is (B^2+(standard deviation of the correction value)^2)^0.5.

[0046] Then, when the standard deviation of this corrected Δt5_6 and the standard deviation (B) of Δt5_6 before correction are kept at the same level, it is necessary to suppress the standard deviation of the correction value sufficiently small compared to B. Therefore, in the ultrasonic flowmeter according to the first embodiment, the average number (N) is adjusted to suppress the standard deviation of the correction value sufficiently small. That is, the average number (N) is adjusted so that the standard deviation of the corrected Δt5_6 and the standard deviation (B) of Δt5_6 before correction are kept at the same level (for example, the difference is within 1%).

[0047] As described above, according to the first embodiment, the ultrasonic flowmeter includes a received signal acquisition unit 401 that acquires a received signal received by the ultrasonic sensor 2, a received signal acquisition unit 402 that acquires a received signal received by the ultrasonic sensor 3, a zero-cross point measurement unit 403 that measures the time from the start of transmission to the zero-cross point multiple times for each of multiple unit measurement steps based on the acquisition results by the received signal acquisition unit 401, a zero-cross point measurement unit 404 that measures the time from the start of transmission to the zero-cross point multiple times for each of multiple unit measurement steps based on the acquisition results by the received signal acquisition unit 402, and a zero-cross point measurement unit 405 that compares the measurement results by the zero-cross point measurement unit 403 with the zero-cross point measurement unit 406. The ultrasonic flowmeter according to the first embodiment includes a time difference calculation unit 405 that calculates a time difference from the measurement result by the measurement unit 404, an average value calculation unit 406 that calculates an average value of the time differences at the first zero crossing points based on the calculation result by the time difference calculation unit 405, an average value calculation unit 407 that calculates an average value of the time differences at the second zero crossing points based on the calculation result by the time difference calculation unit 405, a difference calculation unit 408 that calculates a difference between the calculation result by the average value calculation unit 406 and the calculation result by the average value calculation unit 407 as a correction value, and a flow rate calculation unit 409 that calculates the flow rate of the fluid to be measured based on the calculation result by the time difference calculation unit 405 and the correction value calculated by the difference calculation unit 408. As a result, the ultrasonic flowmeter according to the first embodiment can reduce measurement errors compared to conventional methods even when flow rate measurement is performed using zero crossing points.

[0048] It should be noted that, within the scope of the present invention, any of the components of the embodiments may be modified or omitted. [Explanation of symbols]

[0049] 1 Measuring tube 2. Ultrasonic Sensor 3. Ultrasonic Sensor 4 Arithmetic section 401 received signal acquisition unit (first received signal acquisition unit) 402 received signal acquisition unit (second received signal acquisition unit) 403 Zero-crossing point measurement unit (first zero-crossing point measurement unit) 404 Zero-crossing point measurement unit (second zero-crossing point measurement unit) 405 Time Difference Calculation Unit 406 Average value calculation unit (first average value calculation unit) 407 Average value calculation unit (second average value calculation unit) 408 Difference calculation part 409 Flow rate calculation section

Claims

1. a first reception signal acquisition unit that acquires a reception signal received by one of the ultrasonic sensors; a second reception signal acquisition unit that acquires a reception signal received by the other ultrasonic sensor; a first zero-crossing point measurement unit that measures a time from a start of transmission to a zero-crossing point multiple times for each of multiple unit measurement steps based on an acquisition result by the first received signal acquisition unit; a second zero-crossing point measurement unit that measures a time from a start of transmission to a zero-crossing point multiple times for each of multiple unit measurement steps based on an acquisition result by the second received signal acquisition unit; a time difference calculation unit that calculates a time difference between a measurement result by the first zero cross point measurement unit and a measurement result by the second zero cross point measurement unit; a first average value calculation unit that calculates an average value of time differences at a first half zero crossing point based on a calculation result by the time difference calculation unit; a second average value calculation unit that calculates an average value of time differences at a second zero crossing point based on a calculation result by the time difference calculation unit; a difference calculation unit that calculates a difference between a calculation result by the first average value calculation unit and a calculation result by the second average value calculation unit as a correction value; a flow rate calculation unit that calculates a flow rate of the fluid to be measured based on a calculation result by the time difference calculation unit and a correction value calculated by the difference calculation unit; An ultrasonic flow meter equipped with

2. The first average value calculation unit calculates a final average value by repeating the calculation of the average value N times and averaging the results, The second average value calculation unit calculates a final average value by repeating the calculation of the average value N times and averaging the results.

2. The ultrasonic flowmeter according to claim 1.

3. A first reception signal acquisition unit acquires a reception signal received by one of the ultrasonic sensors; A step in which a second reception signal acquisition unit acquires a reception signal received by the other ultrasonic sensor; a first zero-crossing point measuring unit measuring a time from a start of transmission to a zero-crossing point multiple times for each of multiple unit measurement steps based on an acquisition result by the first received signal acquiring unit; a second zero-crossing point measuring unit measuring a time from a start of transmission to a zero-crossing point multiple times for each of multiple unit measurement steps based on an acquisition result by the second received signal acquiring unit; A time difference calculation unit calculates a time difference between a measurement result by the first zero cross point measurement unit and a measurement result by the second zero cross point measurement unit; A first average value calculation unit calculates an average value of time differences at first zero crossing points based on a calculation result by the time difference calculation unit; A second average value calculation unit calculates an average value of time differences at a second zero crossing point based on a calculation result by the time difference calculation unit; a difference calculation unit calculating a difference between a calculation result by the first average value calculation unit and a calculation result by the second average value calculation unit as a correction value; a flow rate calculation unit calculating a flow rate of the fluid to be measured based on the calculation result by the time difference calculation unit and the correction value calculated by the difference calculation unit; A flow rate calculation method using an ultrasonic flowmeter having the above structure.

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